Magnetic electron -lens



April 8, 1953 R. 5. PAGE 2,637,000

MAGNETIC ELECTRON LENS Filed Feb. 8, 1952 MAGNETIC NON-MAGNETIC NETlC lhventorz Richard S. Page,

by w 4 M His Attorney.

Patented Apr. 28, 1953 MAGNETIC ELECTRON LENS Richard S. Page, Manchester, England, assignor to General Electric Com New York Application February 8, 1952,

pany, a corporation of Serial No. 270,655

In Great Britain February 15, 1951 (01. sis-84 8 Claims.

This invention relates to improvements in magnetic electron lenses, and particularly to electron lenses in which inherent astigmatism may be essentially eliminated.

In instruments, such as electron microscopes and electron diffraction cameras, which employ magnetic lenses, the final image may suffer from astigmatism due to asymmetry in the magnetic field of the imaging lenses. This asymmetry arises from mechanical im erfections and also from nonhomogeneous magnetic properties of the iron which is employed for the pole pieces.

It is known that the field of an astigmatic lens is equivalent to a rotationally symmetrical field with a weak superimposed cylindrical field, and it has been proposed to provide compensation by adding a cylindrical field at right angles to the existing one. For example, in the U. S. Patent 2,455,676, issued December 7, 1948, to James Hillier, soft iron screws are arranged radially in a non-magnetic spacer which is disposed within the magnetizing coil between the pole pieces of the lens. This means of adjustment for compensation involves taking observations with the instrument, e. g. the microscope, in operation, and since in order to adjust the screws the lens has to be completely dismantled or at least dismantled to such an extent that the vacuum within the instrument is destroyed, the correction procedure takes a very considerable time to complete.

The present invention has for its object the provision of correction means which may be adjusted very simply and rapidly if correction is required.

According to the present invention, the improved magnetic lens comprises a magnetic structure which includes a spool or bobbin adapted to receive a magnetizing winding and to be sealed at its end cheeks or flanges with other parts of the vacuum system of the instrument. One end cheek or flange is formed with a plurality of radially directed holes which extend into a non-magnetic member acting to space the pole pieces of the lens but which do not penetrate into the vacuum space within the lens or instrument. The radially directed holes are arranged to receive for adjustment axially thereof rods or the like which, at least at their inner ends, are formed of soft iron or other material appropriate for modifying the magnetic field extending between the pole pieces.

In such arrangement, the position of the rods is adjustable from the outside of the instrument while, since the holes in which the rods are ac- 2 commodated do not extend into the vacuum space, no vacuum seals are required and there is no possibility of any leakage.

Preferably the holes are threaded to receive screwed rods so that by turning the rods, conveniently by means of knobs secured to their outer, and, in this case projecting ends, their position axially of the holes, and thus radially of the bobbin or spool, may be adjusted and the required correction effected.

The features of the invention desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawing, in which the single figure thereof shows a schematic axial section of an improved magnetic lens embodying the invention.

In the form of improved lens shown in the drawing, one end check or flange l of the bobbin or spool 2 carrying a magnetizing winding 3 is formed with a greater wall thickness than is normal and is recessed on its inner face to accommodate a portion 4' of a non-magnetic spacer member 4. A portion 4" of non-magnetic spacer member 4 separates pole pieces 5 and 7 which are mounted in the bore 6 of the bobbin or spool 2. Non-magnetic spacer member 4 may be hermetically sealed to spool 2 by any convenient means, such as silver solder placed along the upper and lower surfaces of junction of portion 4 with spool 2. Pole pieces 5 and I may be retained in a desired position within bore 8 by making them a sliding fit or by soldering them therewithin.

Suitable holes 8 are bored radially inwardly through the check or flange I and into the spacer member 4, terminating for example about from the inner bore 6. The inner portions of each of the holes 8 Within the spacer member 4 are of somewhat smaller diameter as shown so that the inner ends of adjustable rods 9 are slidable fits therein. The adjustable rods 9 are formed, at least at their inner ends, of soft iron or other suitable material, while the outer ends 10 of the rods 9 and the outer portions of the holes 8 are suitably threaded so that, for example by turning a knob H which may be fastened to the protruding outer ends of the rods 9, the position of the inner ends of the rods, which lie in the magnetic field between the pole pieces 5 and 1, imay be adjusted to efiect the required correcion.

In practice, it has been found that eight symmetrically arranged adjustable rods are effective to produce the required compensation of the magnetic field.

While my invention has been described by reference to a particular embodiment thereof, alternative constructions Will readily occur to those skilled in the art. For example, spool 2 may be extended beyond flange l to receive another Wind.- ing and permit a completely symmetrical arrangement of pole pieces in bore 6. I, therefore, aim in the appended claims to cover all such equivalent embodiments as may be within the true spirit and scope of the foregoing description.

What I claim as new and desire to secure by Letters Patent of the United States; is:

1. An electron lens comprising a spaced apart pair of pole pieces having aligned apertures, a non-magnetic spacer member having a portion extending between pole pieces to. form an. aperture aligned with the apertures in said pole pieces, a magnetic structure surrounding said pole: pieces and said spacer member, an externally adjustabie eien-ient extending through said magnetic structure into said non-magnetic spacer member Within. a recess which terminates short of the aperture said spacer member, said adjustable element having at least the inner portion thereof formed of magnetic 1naterial, and an. exciting on said magnetic structure for providing a magnetic which links. pole pieces. to form said electron lens.

2;. An: electron lens as inclaim 1 in which said adjustable element is a rod having "threads thereon adapted engage: threads along the pe riphery of said recess.

3. An electron lens comprising a spaced apart pair of pole pieces having aligned apertures, a non-magnetic spacer member having a per ion extending between said pole pieces to form an aperture. aligned with the apertures in said pole pieces, a magnet-icstructure surrounding said pole pieces and said spacer member, a plurality of externally adjustable elements extending through said magnetic structure and into said non-magnetic spacer member Within respective spaced recesses all of which terminate short of the aperture in said spacer member, said adjustable elements having at least the inner portions thereof formed of magnetic material, and an exciting Winding onsaidmagnetic structure for providing a magnetic which ii-nlzs said pole pieces to form said electron lens.

4-. an: electron lens as in claim- 3 in which said adjustable elements are reds having threadsthereon adapted to engage threads along the periphery of the recesses into which they respectively extend.

5. An electron lens comprising a magnetic structure having a central bore through which electrons may be directed, a pair of spaced apart apertured pole pieces positioned Within the bore of said magnetic structure, a non-magnetic spacer member having an apertured portion extending between said pole pieces and being hermetically sealed within said magnetic structure, an externally adjustable element extending through said magnetic structure and into said: non-magnetic spacer member Within a recess which terminates short of the aperture in said spacer member, said adjustable element having at least the inner portion thereof formed of magnetic material, and an exciting Winding on said magnetic structure for providing a magnetic flux which links said pole pieces to form said electron lens.

6. An electron lens comprising a spool of magnetic material, a pair of spaced apart apertured pole pieces positioned within the central bore of said spool adjacent: one of the end, flanges of said spool, a non-magnetic spacer member having an apertured portion extending between said pole. pieces and being hermetically sealed Within the central bore of said spool, said one end: flange or said spooi being recessed adjacent the central bore to receive a second portion of non-magnetic spacer member, said one end flange of said spool and said second portion of said; non-magnetic spacer member having therein a plurality of aligned radially directed holes which extend from the exterior of; said one end of said spool to near the periphery of the central bore of said spool, radially adjustablerodsv positioned within each of said holes and eaten ing exterioril y of said one end flange to permit adjustment. of the positions oi; saidrods without: disturbing the. remainder, of the electron lens, said rods being of magnetic material at least adjacent. their inner ends, and.

an exciting winding on said spool. for providing magnetic flux which links said pole pieces to form the. electron lens.

7.. an electron lens as in. claim 6. which said rods have a. siiding fit at; least Within iiiner portions of said holes.

8 An electron lens as in claim which said hole. and said rods are threaded in car espondsections; thereof to facilitate the adjustment of said rods Within said holes.

RICHARD PAGE.

Reierences Cited the. file. of this patent.

UNITED STATES PATENTS 

